1. Field of the Invention
The present invention relates to a protective relay for synchronizing the sampling of electrical quantity data at a plurality of terminals on an electric power system by using a Global Positioning System, referred to as GPS.
2. Description of the Related Art
A protective relay system is known in which electrical quantity data, such as electric current, is sampled at a plurality of positions or terminals in an electric power system, such as a power transmission system. Electrical quantity data in digital form obtained by the sampling is exchanged between the terminals and used for calculation for the purpose of providing a protective function for the electrical power system.
In such a system, the sampling at the respective terminals must be synchronized, whereby the time at which the electrical quantity data is sampled at the respective terminals must be made to coincide accurately. This ensures that electrical quantity data obtained by synchronous sampling (that is, simultaneous sampling) is used for the calculation.
There have been proposed various methods for synchronizing the sampling. For example, U.S. Pat No. 4,612,594 issued Sep. 16, 1986 and U.S. Pat No. 4,470,093 issued Sep. 4, 1984, disclose synchronous sampling.
FIG. 1A is time chart illustrating the exchange of synchronous signals between a main terminal SS1 (master terminal) and a secondary terminal SS2 (servant terminal) opposite to each other across a transmission line. That is, a sampling with a sampling frequency T is operated between terminals, and a sampling time deviation xcex94T exists.
First, the terminal SS2 transmits a synchronous signal F0 (sampling synchronized flag) to the terminal SS1 with the electrical quantity data.
On the other hand, the terminal SS1 measures the receiving timing Tm of the data having the synchronous signal F0 as transmitted from the terminal SS2 based on the sampling timing at the terminal SS1. The terminal SS1 transmits the receiving timing Tm and a synchronous signal F1 to the terminal SS2.
Next, the terminal SS2 also measures the receiving timing Ts of the data having the synchronous signal F1 as transmitted from the terminal SS1 based on the sampling timing at the terminal SS2.
Here, it is assumed that the transmission delay time Td from the terminal SS2 to the terminal SS1 and the transmission delay time Td from the terminal SS1 to the terminal SS2 is equal. The transmission delay time Td can be described from the following relationship as between Tm, Ts, xcex94T and T.
Td=Tm+xcex94T+ixc2x7T xe2x80x83xe2x80x83(1) 
Td=Tsxe2x88x92xcex94T+jxc2x7T xe2x80x83xe2x80x83(2) 
2xcex94T=Tsxe2x88x92Tm+(jxe2x88x92i)xc2x7T xe2x80x83xe2x80x83(3) 
xcex94T=(Tsxe2x88x92Tm)/2 xe2x80x83xe2x80x83(4) 
Where i represent an integer (i is equal to 1 in FIG. 1A), and j represents an integer (if Ts is less than Tm, j is equal to i+1, if Ts is Tm or more, j is equal to i.)
Equation (3) is derived from equation (1) and (2). In addition, under the condition that Ts is less than Tm, if (Ts+T) is approximately equal to Ts, then (jxe2x88x92i)xc2x7T in equation (3) may be disregarded. (jxe2x88x92i)xc2x7T is multiple of T. As a result, the equation (3) may be described in the manner as shown in equation (4). Accordingly, the sampling timing at each terminal becomes equal by shifting the sampling timing at the terminal SS2 so that xcex94T can be set equal to a substantially zero value.
FIG. 1B is a time chart illustrating the exchange of synchronous signals between terminals SS1 and SS2. This time chart shows the sampling timing at each terminal is equal. The terminal SS2 measures the sampling frequency T0, which is the time difference between the time point in which the terminal SS2 receives the synchronous signal F0 (sampling synchronized flag) and the time point in which the terminal SS2 receives the synchronous signal F1 (sampling synchronized flag) in return.
The transmission delay time Td is calculated in accordance with the equation (5) based on T0, Ts and T.
Td=T0/2xe2x88x92T+Ts xe2x80x83xe2x80x83(5) 
Accordingly, the delay time of the data transmitted from the terminal SS2 (the opposite terminal) to the sampling timing at the terminal SS1 can be determined.
As mentioned above, by exchanging between terminals the synchronized data, such as electrical data, and the sampling synchronized flag, a protective relay calculation can be made at each terminal by using the sampling data obtained from the same sampling.
The above sampling synchronizing method requires the addition of data for synchronizing into the transmitted data. Therefore, a first disadvantage of this method is that the transmission delay time to both directions on the transmission lines need to be same for establishing synchronous control between terminals.
Furthermore, a second disadvantage of this method as above mentioned is that the rate of operation lowers because the loss of synchronism happens in response to the fluctuation of the transmission delay time even if the downstream and the upstream transmission delay time on the transmission lines are same.
The present invention has been made in view of the above-mentioned circumstances and is intended to solve the above-mentioned problems.
In particular, the object of the present invention is to provide a digital protective relay system in an electric power system, having two or more terminals being coupled to form a pair, and protective relay devices provided at each of the terminals. The system includes receiving sections that respectively are connected to the protective relay device, each receiving section receiving an electric wave from a satellite and outputting a fixed cycle pulse. The system also includes clock signal generating sections that respectively are set in each protective relay device, each clock signal generating sections generating a clock signal. The system further includes sampling pulse generating sections that respectively are set in each protective relay device, each sampling pulse generating section generating a sampling pulse with a fixed frequency in accordance with the clock signal. The system still further includes timing difference measurement sections that respectively are set in each protective relay device, each timing difference measurement section measuring a timing difference of the fixed cycle pulse and the sampling pulse. The system also includes correcting sections that respectively are set in each protective relay device, each correcting section correct a frequency of the sampling pulse from each sampling pulse generating section in accordance with the timing difference. The system further includes digital converting sections that respectively are set in each protective relay device, each digital converting section generating an electrical quantity data in digital form by sampling an electrical quantity of the electric power system in accordance with the sampling pulse of each sampling pulse generating section. The system still further includes transmitting sections that respectively are set in each protective relay device, each transmitting section transmitting and receiving the electrical quantity data between each protective relay device. The system also includes calculating sections that respectively are set in each protective relay device, each calculating section performing a calculation for a protective function by using the electrical quantity data obtained at each protective relay device. The system further includes outputting sections that respectively are set in each protective relay device, each outputting section outputting the result of the calculating section at each protective relay device.